How to Slice an Onion

Slicing onions is easier if the onion has been slightly chilled for about 20 minutes before slicing and your knife is very sharp. Blunt knives put pressure on the onion and encourage the release of the tear producing compounds.
 Also leaving the stem end of the onion on, rather than cutting it off also helps. But when that's not possible, here's how to slice it.

Hold the piece of onion in one hand and a cook or chef’s knife in your working hand.

Gradually bring the knife blade through the onion, removing sections as you go until the entire piece of onion is sliced. The thickness of the slices will depend on the end use of the sliced onion.